<p>A. Three state transition model, which includes a low voltage sensitivity step (C1 – C2) and a high voltage sensitivity step (C2 – O). The rate constants used in the model fitting are: <i>k</i><sub>1</sub> = A1(0).exp<sup>0.0125V</sup>; <i>k</i><sub>2</sub> = A2(0).exp<sup>0.055V</sup>; <i>k</i><sub>−1</sub> = B1(0).exp<sup>−0.025V</sup>; <i>k</i><sub>−2</sub> = B2(0).exp<sup>−0.05V</sup>. The values of A1(0), A2(0), B1(0) and B2(0) for WT and all mutants are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0031640#pone.0031640.s007" target="_blank">Table S5</a>. (i) The simulated unidirectional rates of activation/deactivation at low voltage sensitivity (dashed lines) and high voltage sensitivity (solid lines) compared to the observed rates (filled circles) for WT channels. (ii) The modelled steady-state activation (line) closely fits the observed steady-state activation of WT hERG (filled circles). B (i) Çomparison of the simulated rate constants of activation/deactivation (solid and dashed lines) to the observed rates (filled circles) for V549A. Simulated rate constants were obtained by scaling the WT rate parameters using the ratio of the observed rate constants for V549A compared to WT at <i>V</i><sub>0.5</sub>+40 mV (<i>k</i><sub>1</sub>), <i>V</i><sub>0.5</sub>+130 mV (<i>k</i><sub>2</sub>), <i>V</i><sub>0.5</sub>−40 mV (<i>k</i><sub>−1</sub>) and <i>V</i><sub>0.5</sub>−180 mV (<i>k</i><sub>−2</sub>) (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0031640#pone.0031640.s007" target="_blank">Table S5</a>). (ii) The modelled steady-state activation (line) is similar to the observed state-steady activation (filled circles) of V549A. C. (i) Comparison of the simulated (solid and dashed lines) and observed rates of activation/deactivation (filled circles) for Y545A. (ii) In the case of Y545A, the modelled steady-state activation (line) is very different to that for the observed steady-state activation (filled circles).</p
